1. Field of the Invention
This invention relates to a readout in a radiographic image system. More particularly, it relates to a readout in those radiographic image systems where a radiographic image is recorded on a stimulable phosphor (hereinafter simply called a "phosphor") firstly, then read out, reproduced and at last recorded on a recording-medium as a final image.
2. State of the Art
Up to now, to obtain radiographic images, so-called radiography has been employed which utilizes silver halides. However, recently, for fear of running out of silver resources in the world, it becomes desirable not to use silver halides in picturizing a radiographic image.
A substitute method is developed for the above mentioned radiography where a phosphor accumulating a radiation energy penetrated through an object is stimulated with an energy to emit the stored radiation energy as phosphorescence which is detected and picturized to make an image. As a concrete method, for example, British Pat. No. 1,462,769 and Japanese Patent Publication Open to Public Inspection (hereinafter called "Japanese Patent O.P.I. Publication") No. 29889/1976 propose to convert a radiographic image employing a thermal phosphor as the above-mentioned phosphor, thermal energy as the stimulating energy. This image conversion method employs a panel made by providing a support with thermal phosphor to absorb a radiation through an object thereby accumulates the radiation energy corresponding to the above-mentioned radiation in density, thereafter, by heating the thermal phosphor layer, this accumulated energy is taken out as an optical signal whose amplitude given an image.
Another method for converting a radiographic image is proposed in U.S. Pat. No. 3,859,527 and Japanese Patent O.P.I. Publication No. 12144/1970, which employs an electromagnetic radiation selected from among visible rays and infrared rays as the stimulation energy.
Relating to this method, an improvement of total S/N ratio is proposed in Japanese Patent O.P.I. Publication No. 12429/1970 where a stimulating light having wavelength from 600 to 700 nm is applied and a stimulated emission having 300 to 500 nm wavelength is selectively detected through a photodetector. Because this method requires no heating, the panel in this method is not required to be heat-resistive, and this method is preferable one from this point of view.
However, in this method the stimulating energy is insufficient even when a He-Ne laser is applied, and high-speed scanning on the panel is difficult with the stimulating rays. Moreover, the radiation dose to the object is not sufficiently reduced. On the other hand, because the band gap of the trap which can be stimulated by the rays of 600 to 700 nm wavelength rays is relatively small, a fading phenomenon cannot be ignored and the radiation energy information may be hardly preserved over a long period. Furthermore, with the stimulating rays of this wavelength range, rising time of the stimulated emission fairly lags behind that of the stimulating rays, similarly about the falling time. This makes another factor which retards high-speed scanning on the panel with the stimulating rays.